Implantable medical devices can be used to provide pacing therapy to patients who have cardiac rhythm problems. For example, an implanted medical device can be used to provide pacing therapy to a patient with sinus node dysfunction, where the heart fails to properly initiate depolarization waves, or an atrio-ventricular conduction disturbance, where the conduction of depolarization waves through the heart tissue is impaired.
Implanted medical devices with pacing functionality, such as a pacemaker, typically deliver a pacing pulse of electricity to the heart in order to produce a heartbeat at the correct time. The implanted medical device includes electronic circuitry that is contained within a hermetically sealed enclosure that is sometimes referred to as a pulse generator. The pulse generators and associated electronics are implanted in the patient's chest and one or more leads are routed from the pulse generator, through the patient's vasculature, and to the patient's heart tissue. Electrical pulses are delivered through the leads to the heart tissue, initiating contraction of the heart.
One issue associated with cardiac pacing therapy is the need to adapt the pacing rate in response to the changing metabolic demands of the patient. For example, while a patient is sitting, sleeping, or otherwise being sedentary, the patient's cardiac output requirements are relatively low. However, when engaged in physical activity, a patient's cardiac output requirements increase in order to transport more oxygen to, and carbon dioxide from, various body tissues. The greater the intensity of the physical activity, the greater the cardiac output required to sustain the activity.
Methods have been devised for adapting cardiac pacing rates in response to exercise or exertion, referred to as “adaptive rate pacing” or “rate adaptive pacing”. These methods generally depend on measuring something that serves as an index of exertion and then adjusting the pacing rate in response to changes in the measured quantity. In many cases, some type of algorithm is used to calculate a pacing rate based on exertion data as input. The algorithm may include various rate response parameters to control aspects of how the pacing rate is actually set.